Electrochemical sensors can be, for example, potentiometric or amperometric sensors. An established embodiment of an electrochemical sensor of the field of the invention is, for example, a potentiometric, single-rod, measuring chain for the electrochemical measuring of pH values in many fields of chemistry, environmental analytics, medicine, industry and water management. Such an electrochemical sensor unites a measuring electrode and a reference electrode in one assembly. Utilized as reference electrode is, as a rule, a silver/silver-chloride electrode, which forms a constant potential. This silver/silver-chloride electrode is immersed in a reference electrolyte, for example, KCl-solution, which is located in an annular space around the measuring electrode. Depending on the embodiment, the annular space can contain, besides the reference electrolyte, also one or more bridge electrolytes. The electrolyte, which is in contact with the measured medium at the liquid junction, is referred to in the following as the fill electrolyte. The measuring electrode includes, usually, a glass tube, which is closed facing the medium to be measured with a glass membrane and which is filled with a buffer solution of known pH-value and known chloride ion activity. For sensing the pH dependent potential, likewise a silver/silver chloride wire is immersed in the buffer solution. The Ag/AgCl-electrode forms with the buffer solution a first galvanic half cell, which is in contact with the measured medium via the glass membrane, at which a pH-dependent potential forms.
An amperometric sensor can comprise, for example, a three-electrode circuit with a working electrode, a counterelectrode and a non electrical current bearing, reference electrode. The reference electrode, through which electrical current does not flow, here and in the following likewise referred to as a half cell, can be embodied in an equal manner as the previously described Ag/AgCl-reference half-cell.
The glass tube with the measuring electrode is surrounded by an annular space containing the reference electrode. In a pH-sensor, e.g. a pH, single-rod, measuring chain, usually a silver/silver chloride, reference electrode is used, in order to obtain a reference potential. This Ag/AgCl-electrode forms with the reference electrolyte with known chloride ion activity a second galvanic half cell, which is in contact with the measured medium via an ionically conducting contact, a so-called liquid junction or diaphragm. The liquid junction is placed in the glass wall of the glass container and is composed, for example, of a porous ceramic material, at which charge exchange can occur. This charge exchange is referred to as electrolytic contact.
Due to the character of the liquid junction, measured medium can penetrate into the annular space, and fill electrolyte contained in the annular space can escape from the glass container into the measured medium. This can lead to bleeding or poisoning of the reference electrode, or to the forming of disturbing, diffusion- and streaming potentials through impurities or plugging at the liquid junction, which corrupt the pH-measuring. The bleeding can lead to depletion of the potential determining species in the reference half-cell. A further problem caused by the character of the liquid junction can be that the sensor dries during a long, dry storage. There are therefore special requirements for the storage of such sensors; for example, it must be assured that the liquid junction is immersed during storage in a liquid, for example, a KCl-solution.